The recognition of antigen-presenting MHC class II molecules by CD4+ T cells is a crucial component of the immunological response. Class II molecules, like other transmembrane proteins, are translocated into the endoplasmic reticulum after synthesis, where they associate with a third protein, the invariant chain (Ii). This molecule is a type II transmembrane protein that serves as a class II-specific chaperone which promotes the exit of class II-Ii complexes from the endoplasmic reticulum and prevents class II molecules from binding peptides and unfolded proteins in the endoplasmic reticulum and in the secretory pathway.
A targeting motif in the cytoplasmic tail of Ii directs the complexes from the secretory pathway into the endosomal system. Before the MHC class II molecules can present antigen the Ii must be removed. This is accomplished by a series of proteases that break Ii down into small peptides. However, an Ii fragment, called class II-associated invariant chain peptide (CLIP), which occupies the peptide-binding groove of the class II molecule, is in most cases not spontaneously released. The CLIP fragment serves as a substitute peptide that protects the class II binding pocket from collapsing both during intracellular transport and after Ii degradation in the endosomal system. Binding of antigenic peptides, generated from endocytosed proteins, requires an empty, yet open binding site, and therefore CLIP has to be released while the open binding site needs to be stabilized to allow the binding of other peptides. Human Leukocyte Antigen DM (‘HLA-DM’) has been well documented to mediate both of these functions, thus promoting the binding of antigenic peptides. After acquiring peptides, the class II molecules are transported to the cell surface via routes that are largely unknown.
Blocking the presentation of antigens is a promising way to inhibit the immune response. This could be done by disrupting the uptake, the proteolytic processing, or binding to MHC class II molecules. Blocking the uptake may be problematic since many different cell types require this function. Inhibition of the proteolytic processing of particular antigens may be of use since different proteases may be involved in cleaving different antigens, however these proteases are not specific and may lead to other side-effects. One way to specifically block the binding to the antigens to the MHC class II is to inhibit the proteolysis of the invariant chain. If this is not removed then the MHC class II molecules cannot be loaded with peptides, hence blocking Ii degradation would decrease antigen presentation to CD4+T-cells and disrupt the normal immune response.
Cathepsin S (CatS) is a cysteine protease expressed in lymphatic tissues. It is has been identified as playing a major role in invariant chain proteolysis which is a prerequisite for peptide loading of MHC class II (Riese et al. (1996) Immunity 4:357). It has 50–60% identity to cathepsins L and K, but differs in that it has a broad pH optimum that extends to alkaline pH. Inhibitors have been shown in animal models to modulate antigen presentation and are effective in an asthma model (Riese et al., J. Clin. Invest. (1998) 101:2351). Mice deficient in cathepsin S have an impaired ability to present exogenous proteins by professional antigen presenting cells (Nakagawa et al., Immunity (1999) 10:207; Shi et al., Immunity (1999) 10:197).
Compounds that inhibit the proteolytic activity of human cathepsin S are expected to find utility in the treatment of chronic autoimmune diseases including, but not limited to, lupus, rheumatoid arthritis, and asthma; and have potential utility in modulating the immune response to tissue transplantation.
Methods of modulating autoimmunity with an agent that modulates cathepsin S activity, e.g. proteolysis of the Ii chain, as well as methods of treating a subject having an autoimmune disorder, methods of evaluating a treatment for its ability to modulate an immune response are described in WO 99/58153.